A study of structure and dynamics of polyelectrolyte solutions using flow birefringence measurements

Abstract

Stress optical data from polyelectrolytes (sodium polystyrenesulfonate) in aqueous solutions have been determined using flow birefringence. The stress optical rule was found to be violated in the semidilute unentangled concentration regime but was found valid in the I semidilute entangled regime for shear rates above 100 s-1. It is shown that the stress optical rule violation in the semidilute entangled regime occur as a result of spatial inhomogenity. In the sentidilute unentangled regime, the stress optical rule fails from both spatial inhomogenity and excessive stretching of the polymer chain. In addition, the orientation of polyions were found to be strongly shear rate dependent for all concentrations. In stress relaxation measurements, the relaxation times were observed to be independent of concentration in the semidilute entangled regime (>IO mg/ml). In the semidilute unentangled regime, the relaxation times were highly dependent on concentration and scales as c-0.5 and this behavior was observed over two decades of concentration (0.05mg/ml-I mg/n-fl). In steady shear experiments, the viscosity-versus-shear rate curves manifest at least three distinct flow regimes that are sin-tilar to those observed in textured polymer liquid crystals. Thus, in analogy to liquid crystal polymers where excluded volume interactions cause texturing, we show for the first time that iv electrostatic interactions in polyelectrolytes can cause domain formation. As in liquid crystalline polymers, the domain size and uniformity appears to depend on molecular weight and concentration. Steady shear experiments also show that the viscosity scales as q c 112 Ml-oin the semidilute unentangled regime and as q c3l2M3 in the entangled semidilute regime, in agreement with a recent theory by Dobrynin et all. The relaxation time, however, scales as ,r M-IC-112 in the semidilute unentangled regime and as T-Moco in the semidilute entangled regime. This last observation is at odds with scaling theory that suppose reptation-like dynamics in the semidilute entangled regime. Rather, we propose that polyelectrolyte chains at these high concentrations interact to form a virtually permanent network of isotropic chains. When the network is deformed, the chains react by instantaneously snapping back to their original length. The addition of salt to the polyelectrolyte solution could disrupt this network and thus cause a recovery of reptation diffusion, as in solutions of uncharged entangled polymers.